That approach is ephemeral though - you still have to then update your build config if you like it.
So it is. However, I find it easier to think "now where was that text file that holds my list of desired packages" than "now which package manager was I using in this machine? Was it Alpine/apk or Ubuntu/apt" - and I haven't used any RPM-based distros for decades, now, so I'd have to look up a newb guide for these...
upstream management is quite a bit slower, even on unstable
Well, yeah, if you have to recompile a package every time, then it's gonna take time :) I can't see how it could be faster on Arch, though. Unless your package also pulls in beta dependencies, so you'll have a whole dependency tree to recompile.